🚀 go-pugleaf

From time to time one sees questions about programs to turn dates 
into days elapsed, days of the week, etc.  For those who are
inclined to develop a "generic" program to do this sort of thing
over a long period of time, here are a few complications to
consider (Ref: The New Encyclopaedia Britannica 15th Edition 1983):

In the 16th Century, Pope Gregory XIII introduced the so-called
Gregorian calendar to replace the earlier Julian calendar.  The
purpose was to recalibrate to calendar year with the celestial
year so that the vernal equinox would once again fall on 21 March
as it had at the time of the Council of Nicaea in AD 325.  The
major effect was the "loss" of 11 days in October 1582 -- it was
proclaimed that the day following 4 Oct 1582 would be 15 Oct 1582.
And, apart from some rioting in the streets and similar statements
of concern by the great unwashed at the "loss" of 11 days from 
their lives, this change was put into effect.

Unfortunately, the wider world didn't immediately fall into line
with this papist decree.  In fact, some delayed their response by
a Century or two.  The introduction of this geopolitical variation
will further complicate attempts at "generic" programming of dates.

For those who still harbour such ambitions the following data may
be helpful:

Year of transition to the Gregorian calendar:

1582	Italian states, Portugal, Spain
1583	Catholic German states
1699	Protestant German states
1752	England and colonies
1753	Sweden
1873	Japan
1912	China
1918	Soviet Union
1923	Greece

Apart from dropping a few days, the Gregorian calendar only differs
from the Julian in that no century year is a leap year unless exactly
divisible by 400 (i.e. the year 2000 will be a leap year).

A further refinement, years evenly divisible by 4000 are NOT leap
years, will keep it accurate to within 1 day in 20,000 years.

Oh, and don't forget that Muslim countries tend to retain calendars
based on Islam.  :-)


Cheers, and may your algorithms be concise,

Ian S.

-- 

Ian Staples                        E-mail : I.Staples@dpi.qld.gov.au
c/- P.O. Box 1054 MAREEBA          Phone  : +61 (0)70 928 555 Home 924 847
Queensland Australia 4880            Fax  : +61 (0)70 923 593   "   "   "